Gases in street manholes

Abstract

Marine sand is one of the most important marine mineral resources in the Pearl River estuary as well as nearby Hong Kong waters of China. For more healthy and order development of marine sand extraction activities, about 518 km of initial seismic exploration and marine drilling of 5 validation boreholes with total footage of 176.7 m were carried out in 2001 and 2002 respectively, and the preliminary overall situation of marine sand resources in the Pearl River estuary waters was clarified. Based on geological interpretation of GeoPulse seismic profiles and core sample analyses of boreholes, the Quaternary stratigraphic sequence in the Pearl River estuary was divided into four depositional units: the upper marine unit of Holocene marine mud, and other three late Pleistocene units: the upper terrestrial unit of weathered mottled clay, the lower marine unit of silty clay, and the lower terrestrial unit of alluvial sand. The main type of marine sand occurrence and the main exploration targets for marine sand resources are identified in the late Pleistocene paleo-Pearl River channel sand at the lower terrestrial unit. The most valuable marine sand resources are located in Holocene tidal channel which coincide with late Pleistocene paleo-channels where the Holocene marine mud overburden is thinner. Sustainable development and scientific management of marine sand resources in the Pearl River estuary waters must strengthen legal system construction, work out scientific exploitation planning, strengthen environmental impact research, and strictly enforce pertinent laws. In about 3000 km2 Pearl River estuary waters tentative estimation of reserves of buried marine sand is 12.5 × 108 m3 in 20 sites, of which 10 sites with 5.38 × 108 m3 were suggested to be suitable for current exploitation, other part of these reserves were put aside for exploitation as their thick cover in 4 sites with 2.75 × 108 m3, seriously contaminated cover in 1 site with 2.5 × 108 m3 or ecological or environmental sensitivity in 5 sites with 5.38 × 108 m3. This research results can give some overall description of marine sand resources in the Pearl River estuary waters and provide scientific base for macroscopic management. Gassy sediments recorded by acoustic turbidity change the geoacoustic properties of the seabed, weaken geophysical exploration effect, and have great engineering and environment significance.